A telescope has a main mirror, also called primary mirror. The primary mirror concentrates the light rays toward a secondary mirror which reflects them back to the focal point of the telescope. The primary mirror must not deform under the effect of gravity for example. Often, an intermediate deformable mirror is used to correct the defects of the primary mirror. And the intermediate mirror is deformed by one or more actuators.
For an active optics application, an actuator with a very high precision and stability is sought. It may even be desirable to have a nanometer-level precision, in other words of the order of a nanometer. Ideally, the actuator must operate at best around its initial position. This is what is also referred to as having symmetrical travel around the mechanical zero. Lastly, its coefficient of expansion, denoted CTE in the literature, must be as low as possible.
In various fields of application, precision actuators are required. From amongst the precision actuators, piezoelectric actuators may be mentioned.
The direct piezoelectric effect is the property according to which the application of a mechanical load to certain crystals or ceramics causes electrical charges to appear on the surface of the material. The direct piezoelectric effect may be exploited in the design of sensors such as pressure sensors.
The inverse piezoelectric effect is the property of deformation of a piezoelectric material when an electric field is applied to it. The inverse piezoelectric effect allows actuators to be designed.
There exist a very large number of piezoelectric materials. The most well known is quartz. However, it is the synthetic ceramics PZT (for lead zirconium titanate, also known as LZT in the literature) that are just as widely used today in the industry.
There exist two main types of piezoelectric actuators. The first type of actuator is called a direct actuator, in which the displacement obtained is equal to the deformation of the piezoelectric material. Direct actuators allow a travel of between 0 and 100 micrometers to be obtained. The second type of actuator comprises amplified actuators, in which a mechanical device amplifies this movement by a factor of 2 to 20. Amplified actuators generally have a travel in the range between 0.1 mm and 1 mm.
Today, it is multilayer ceramics (also known as MLA for Multi-Layer Array in the literature) that are conventionally used in piezoelectric actuators. The integration of this type of material imposes specific precautions. The necessity to provide a mechanical pre-stressing or to avoid torsion forces may in particular be mentioned. With the proviso of a good design and implementation, piezoelectric actuators are extremely reliable and robust.
Their reliability and robustness have enabled piezoelectric actuators to be used in the field of space applications. They are also used, for example, for nanopositioning, the creation of vibrations, and the active control of vibrations.
Today, aside from the field of space applications, piezoelectric actuators are used in several areas. The following may notably be mentioned:
in industry for machining assistance by creation of vibrations;
the control of certain injectors in the automobile industry carried out by virtue of piezoelectric materials. This technique notably allows the process of fuel injection to be well controlled;
some inkjet printers using piezoelectric elements for producing the fine droplets which are propelled onto the paper.
Currently, a piezoelectric actuator with pre-stressing is used to deform the intermediate mirror. At rest, the actuator is said to be at its initial or reference position, also referred to as the position of the mechanical zero. The travel of such an actuator is asymmetric. For example, the actuator has a travel in the range between −5 μm and +40 μm. The difficulty resides in the center-shift of the travel which implies having a significant offset of the voltage in the central position. In this case, the initial position is no longer the desired mechanical zero.
Another solution consists in using two actuators connected in opposition (also referred to as “push-pull” in the literature) where their forces are added together. Each actuator must deform the other when it is actuated. This solution only allows a limited travel. More precisely, the actuators have a complementary displacement. However, the asymmetry of the displacement leads to a residual force at the mid-point or at rest. The series push-pull doubles the force for a constant travel.
Thus, it is observed that the use of a piezoelectric actuator alone does not allow a desired symmetrical travel to be obtained. It is necessary to pre-stress the system and to offset the mechanical zero. This renders problematic a potential case of failure where the actuator gets blocked in an end position.
The use of an actuator in “push-pull” mode is a known and advantageous solution. Nevertheless, it reduces the total travel of the actuator and imposes the use of large actuators in order to obtain the desired travel.
Lastly, the use of an actuator with a micromotor, a reducer and a screw allowing a de-multiplication to be obtained is advantageous. Nevertheless, the de-multiplication increases the requirement in travel of the actuator which may then become too large. This type of actuator cannot therefore be envisioned for a space application.